1. Field of the Invention
The present invention relates to hearing aids and to ossicular prostheses, and, more particularly, to a prothesis used in conjunction with a hearing aid using magnetic induction to reproduce sound.
2. Description of the Prior Art
Hearing aids are useful in restoring lost aural perception to those persons having mild to severe loss of hearing. Conventional hearing aids have a microphone, amplifier circuitry, a battery and a speaker. The microphone receives the sound energy and transforms the sound energy into an electrical signal which is then amplified and filtered. This amplified signal is transformed back to acoustic energy by the speaker and transmitted to the person's middle ear for perception of the sound. These hearing aids can be placed behind the ear, with only the receiver being placed inside the ear canal. Alternatively, in-the-ear hearing aids are available which are placed in the outer ear and have portions extending into the ear canal.
There are a number of problems with conventional hearing aids. All conventional hearing aids are visible to some extent and therefore have an undesirable cosmetic appearance. Conventional hearing aids have acoustic feedback problems because sound energy can escape from the ear canal and be detected by the microphone, generating a feedback-related whistle. Additionally, sound reproduction is often lacking in clarity because of distortions generated by standing waves existing in the closed cavity between the hearing aid and the tympanic membrane and poor mechanical reproduction by the speaker.
It has been suggested that a magnetic induction hearing aid would remove many of these problems. A magnet or other item having a magnetic field is placed in the middle ear, either in contact with the tympanic membrane or in contact with other portions of the middle ear. Electrical circuitry and a coil would generate a magnetic field having the same frequency as the external sound. The magnetic field generated by the coil would interact with the field of the magnet and cause the magnet to vibrate at the same frequency as the magnetic field. The vibration of the magnet would then cause the attached portion of the middle ear to vibrate, resulting in a perception of the external sound.
A magnetic induction hearing aid would overcome feedback or distortion problems of conventional hearing aids because there would be no significant air movement in the ear canal, resulting in insufficient energy escaping around the hearing aid to generate a feedback problem. There would be no standing waves generated to cause distortion because there are no appreciable sound waves at all.
Attempts to use magnetic induction hearing aids have been reported. An early attempt placed a coil in conjunction with a small piece of iron on the tympanic membrane, which was excited by an external coil placed over the ear canal. This system did allow the perception of the stimulus, but had the side effect of producing discomfort and pain for the wearer. A later attempt glued a small magnet to the umbo and used an external coil placed over the ear of the wearer to cause the sympathetic vibrations of the magnet. This apparatus required approximately 7.9 ma to produce a 0 db hearing level at 1000 Hz.
In an article entitled Audition via Electromagnetic Induction, Arch Otolaryngol 23 (July 1973), Goode et al describe a number of tests. One test attached a magnet to the tympanic membrane and located a coil in the ear canal 3 mm from the magnet. The coil was driven externally by an audiometer. This development required only 0.7 .mu.a to produce a 0 db hearing level at 1000 Hz. Tests were performed for system fidelity and proved adequate. Another system tested placed the coil over the ear, drove the coil with an audiometer and had a magnet glued to portions of the middle ear, but used larger magnets than in previous tests. One version of this system placed the magnet on a Silverstein malleus clip which was connected in the normal manner. Approximately 0.7 ma was required to produce a 0 db hearing level using these arrangements.
These discussions suggested that the use of electromagnetic induction to produce a hearing aid is possible, but did not teach a way to develop a practical system. The majority of tests used coils placed over the ear or adjacent to the ear. Systems using external coils are not efficient enough for use in conjunction with the low power requirements dictated by hearing aid batteries. Although one test indicated that a coil was placed inside the ear canal, an external amplifier was used to drive the coil. The tests did not result in a practical device or suggest how a totally in-the-ear device could be made.
Further, the magnets described in conjunction with the above-mentioned tests were either glued to portions of the middle ear and removed after short periods of time or were connected to malleus clip and inserted for a longer duration. Neither of these attempts resulted in a magnet that could be implanted for extended periods of time with no danger of rejection by the body, have no movement in relation to the middle ear and yet have as little weight as possible.
In a number of people, portions of the ossicular chain have been damaged due to trauma or other reasons and therefore a hearing loss occurs. One solution to this problem has been the use of ossicular prostheses to replace the damaged portions of the ossicular chain. The prostheses include total ossicular replacement prostheses and partial ossicular replacement protheses depending upon the damage to the individual ossicular chain. In addition, many of the patients with damage to portions of the middle ear also have damage to the sensorineural portions of the ear contained in the cochlea. The placement of a prosthesis allows an individual to regain significant amounts of the hearing that was lost due to the damage to the ossicular chain, but in a majority of cases there is still a residual hearing loss. Correction of the residual hearing loss has typically been performed by using traditional hearing aids which have the problems discussed above. Therefore, although the addition of a prosthesis is a large step in solving a hearing loss developed due to damage of the ossicular chain, conventional hearing aids must still be used to bring hearing levels up to that existing prior to damage to the middle ear.